swarm: fix external address candidate

The `NewExternalAddrCandidate` event is yielded both before and after address translation. This will
cause, in the case of TCP, ephemeral ports to be added as candidate.

In turn, that will cause protocols like AutoNAT to fail as these candidates are not actually
reachable/external.

Fixes #4153

swarm/src/lib.rs

@@ -1062,11 +1062,6 @@ where
                 self.pending_event = Some((peer_id, handler, event));
             }
             ToSwarm::NewExternalAddrCandidate(addr) => {
-                self.behaviour
-                    .on_swarm_event(FromSwarm::NewExternalAddrCandidate(
-                        NewExternalAddrCandidate { addr: &addr },
-                    ));
-
                 // Generate more candidates based on address translation.
                 // For TCP without port-reuse, the observed address contains an ephemeral port which needs to be replaced by the port of a listen address.